Simplify c-stack prerequisites. * lib/c-stack.c (includes): Remove unused <sys/resource.h>. * m4/c-stack.m4 (AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC): Posix 200x no longer requires <ucontext.h> to exist. Optimize setrlimit check. (gl_PREREQ_C_STACK): Remove check for unused getcontext and <sys/resource.h>. Signed-off-by: Eric Blake <ebb9@byu.net>